For holding trays with eight resilient mounting fingers there is a device described in PCT application WO 94 05010. This device is not fit for use in a holder tray with twelve mounting fingers because the space between the mounting fingers is limited especially towards the center. While the spring has to be strong enough to lift the disk, there is a limit to the width of the spring. Mounting the device is impossible because the spring will not pass between the fingers and would be bent and thus rendered useless.
For holding trays with 12 mounting fingers there is a device known from European patent application EP-A-429195. A four-armed spring urges the disk upwards when the mounting fingers are pressed, This publication describes also a push button with pivoting arms. Both applications have the arms resting upon the resting surface of the container, resulting in less gripping surface of the mounting fingers and only a four point resting of the disk. This could lead to deformation of the disk.